Solid composite propellant containing acrylamide polymers



United States Patent 3,000,713 SOLID COMPOSITE PROPELLANT CONTAINING ACRYLAMIDE POLYMERS Marvin H. Gold, Pasadena, Calif., assignor to Aerojetgfileral Corporation, Azusa, Calif., a corporation of o No Drawing. Filed Nov. 16, 1953, Ser. No. 392,472 11 Claims. (Cl. 52-.5)

This invention relates to solid propellant compositions. In particular this invention relates to novel, smokeless propellant compositions of high specific impulse comprising a finely divided oxidizing agent uniformly dispersed in an amide type resin.

Solid propellant compositions are ordinarily composed of a resin fuel and an oxidizing material, the oxidizer being intimately dispersed in the fuel. The fuel component usually comprises a polymeric organic resin such as a polyester and the oxidizer usually is a salt such as ammonium perchlorate. The physical ignition and burning properties of such a propellant composition are dependent upon the ratio of oxidizer to fuel, the type of mixing employed and the particle size.

Heretofore it has been difficult to incorporate a high concentration of oxidizer in propellants without reducing the quality of the casting properties of the grain. Moreover, it has been extremely difiicult to obtain a uniform composition throughout the propellant grain. Variations in composition produce internal strains responsible for cracking of the grain, and cause erratic burning of the propellant. It has also been difiicult to obtain an efficient propellant without incorporating into it a metal-containing oxidizing salt. Formerly, propellants which did not contain metal-containing oxidizing salts were difficult to ignite and did not adequately sustain combustion. The combustion of the metal containing salts however, produces undesirable smoke due to the formation of undesirable metal oxides.

By means of the present invention, it is possible to obtain propellant compositions in which a high concentration of oxidizer may be incorporated without accompanying degradation of the casting properties of the polymer, moreover these propellant compositions have an improved distribution of the oxidizer throughout the fuel component, and permit the use of smokeless non-metallic oxidizing salts. As a result of these features the propellant grain thus obtained exhibits a more uniform burning rate, improved physical and ballistic properties, and burns in a smokeless manner.

In accordance with the present invention it is proposed to employ polymeric resin fuels derived from monomers in which an oxidizing salt is soluble in amounts in excess of 5% by weight of the composition prior to polymerization. The oxidizing agent is thoroughly mixed with the monomer and partially or completely dissolved, thereby providing a uniform distribution of the oxidizer throughout the fuel. The mixture is then heat polymerized in the presence of a catalyst to produce the propellant grain.

The polymerizable monomers found to be useful for this invention are organic products of the ammonolysis of an organic acid halide with ammonia or an amine, in which at least one reactant contains a group having an olefinic linkage. The monomers are polymerizable amides and urethanes containing an olefinic linkage, having an oxygenzcarbon weight ratio of less than 1 to 6 and may be represented by the general formula:

wherein R is a hydrogen alkenyl or alkenyloxy group;

Patented Sept. 19, 1961 R is a hydrogen, lower alkyl, lower alkenyl or nitro group; and R" is a hydrogen, lower alkyl or lower alkenyl group, wherein at least one R is an unsaturated group. They may be used either individually or as mixtures. Monomers which have been found to be particularly useful in this invention are N,N-dimethyl acrylamide, N- methyl acrylamide, N-nitro-N-methyl acrylamide, acrylamide, N,N-diallyl formamide, and N,N-dimethyl vinyl carbamate. N,N-dimethyl acrylamide, N-methyl acrylamide and diallyl formamide are prepared by condensing the amine with the appropriate acid halide. N-nitro-N- methyl acrylamide is then prepared by the nitration of N-methyl acrylamide with nitric acid. N,N-dimethyl vinyl carbamate is similarly prepared by allowing the amine to react with the halide salt of the unsaturated oxyacid, namely vinyl chloro carbamate.

The oxidizing agents found to be suitable in this invention are the amine salts of perchlorates, nitrates, and chlorates, such as the ammonium, methyl ammonium, dimethyl ammonium, trimethyl ammonium, methyl anilinium, anilinium dimethyl, anilinium, tetramethyl ammonium, triethyl ammonium, benzyl dimethyl ammonium, benzyl trimethyl ammonium, and benzyl trimethyl ammonium salts. The oxidizing material may be composed of a combination of oxidizers without departing from the scope of this invention. The amount of oxidizing agent which may be incorporated in the dispersion may vary from about 5% to about of the total weight of the propellant composition. When the dispersion is to be used as a high impulse rocket propellent, it is preferred that the oxidizing agent be present in an amount of from about 50% to 80% of the total weight of the propellant composition.

Compounds such as styrene, methyl acrylate, acrylonitrile and other polymerizable olefins and such materials as unsaturated, polymerizable alkyd resins may be incorporated in the polymerization mixture in minor quantities, if desired, without departing from the scope of this invention. These compounds are well known to be useful in propellant compositions and provide a source of combustible material, in addition to that found in the principal monomer. However, it is apparent that as the concentration of these compounds in the fuel is increased the solubility of the oxidizer in the fuel is decreased. T herefore, in order to maintain a substantial amount of dissolved oxidizer in the polymerization mixture, it is preferred that these additional materials do not constitute more than 25% by weight of the fuel component.

The dispersions provided by the present invention are prepared by the following method. An oxidizing ammonium or substituted ammonium salt is thoroughly mixed in at least one polymerizable amide or urethane monomer wherein part or all of the oxidizer is dissolved. The resulting mass is then heat polymerized. As the fuel polymerizes, the dissolved oxidizer is progressively forced out of solution and deposited uniformly throughout the grain in a finely divided condition. The oxidizer which was not dissolved in the monomer but intimately mixed with it, is occluded in the fuel as the mass solidifies. The polymerization is usually carried out at a temperature in the range of from about 25 C. to about C. and preferably at a temperature below about 60 C. Polymerization catalysts may also be used if it is desirable to reduce the casting time. Lecithin in concentrations greater than 1% by weight of the composition have been found to improve the casting properties of the composition.

To obtain optimum results in the preparation of the propellant grains of this invention it has been found that the particle size of the solid oxidizing agent to be employed should be carefully regulated since this factor determines to some extent the casting and molding properties of the resulting polymeric dispersion. The optimum blend of the particle size of the solid oxidizing agent is determined experimentally for the particular dispersion desired. For example, when compounding a castable propellant, consisting of a high concentration of ammonium nitrate dispersed in polydimethyl acrylamide, it has been found that a particle size blend comprising 70 weight percent of ammonium nitrate screened through a mesh screen and retained by a 20 mesh screen and 30 weight percent of ammonium nitrate of a particle diameter of less than 840 microns yields optimum results.

The mixture of oxidizing agent in the monomer or monomers is subjected to polymerization by adding 0.05% to 0.5% of a polymerization catalyst; the polymerization catalysts usually employed for this purpose are organic peroxides such as benzoyl peroxide, lauryl peroxide, acetobenzoyl peroxide, ditertiary butyl peroxide, methyl ethyl ketone peroxide, l-hydroxy cyclohexyl hydro-peroxide, cumene hydroperoxide, cycloalkane hydrocarbon peroxide and peresters such as tertiary butyl perbenzoate and diperphthalate. In some cases however, the dissolved oxidizing agent itself may act as a polymerization catalyst.

The following examples are furnished to more clearly described the present invention. It is to be understood, however, that these examples are provided purely as a means of illustration and the scope of the invention is not intended to be limited by them.

EXAMPLE I To illustrate the properties of solutions formed by dissolving an oxidizing ammonium salt in a monomer in accordance with my invention, the freezing-point data for the ammonium nitrate-N,N-dimethyl acrylamide system is presented in Table I.

Solutions containing to 19% ammonium nitrate could not be crystallized at temperatures as low as 75 C. As is apparent from tabulated data, the temperature coefficient of solubility of ammonium nitrate in N,N- dimethyl acrylamide is large and the solutions produced are stable for prolonged periods of time and hence may be prepared and then stored if desired.

EXAMPLE II To illustrate the solvent action of a monomer typical of my invention upon typical oxidizing ammonium salts, the following solubility data is presented. The determinations were made at a temperature of 25 C.

Table II Solvent Solubility,

Solute weight percent Ammonium nitrate N,N-dimethyl acrylamide. Ammonium perchlorate. d

Approx. 25%. Approx. 32%.

As is apparent from the above tabulation oxidizing ammonium salts are readily soluble in the monomers described in my invention and form stable solutions which may be easily cast and polymerized.

4 EXAMPLE m A propellant having the following formulation was prepared:

Component Weight percent N ,N-dimethyl acrylamide 22. 94 Ammonium nitrate 76. 81 Catalyst, methyl amyl ketone peroxide- 00. 25

The propellant was cured by subjecting it to a temperature of F. for a period of 16 hours. The cured propellant grain exhibited an auto ignition temperature of 460 F. and an impact stability in excess of 100 cm. with the Bureau of Mines 2 kg. apparatus. The burning rate was determined by burning one inch strands of the grain Through the use of solvent-type monomers, polymerization mixtures are obtained which have a relatively low viscosity and therefore are easily transported and cast. Furthermore, the low viscosity of the mixture provides a more even distribution of the components and upon polymerization yields an improved grain, less susceptible to cracking than heretofore has been possible. Moreover, the propellants herein described burn uniformly and in a smokeless manner. In addition, the propellants are stable, can withstand rough handling and perform satisfactorily at temperatures ranging from about 65 F. to about F.

I claim:

1. A smokeless solid propellant composition which comprises a cured intimate mixture of from about 5% to about 95% of a solid inorganic oxidizing salt selected from the group consisting of ammonium, alkyl ammonium alkylaryl ammonium, alkylaryl alkyl ammonium, anilinium, and alkylanilium perchlorates, chlorates, and nitrates; and from about 95 to about 5% of a polymerizable monomer having the formula:

II R-C-III wherein R is a radical selected from the group consisting of hydrogen, lower alkenyl, and lower alkenyloxy; R is a radical selected from the group consisting of hydrogen, lower alkyl, lower alkenyl, and nitro radicals; and R" is a radical selected from the group consisting of hydrogen, lower alkyl, and lower alkenyl groups; at least one R being an unsaturated radical.

2. The composition of claim 1 to which has been added a polymerization catalyst selected from the group consisting of organic peroxides and peresters.

3. The composition of claim 1 wherein the oxidizing salt comprises from about 50% to about 80% by weight of the total propellant composition.

4. A smokeless solid propellant composition according to claim 1 which comprises a cured intimate mixture of ammonium nitrate and a polymerizable monomer having the formula:

wherein R is an alkenyl radical and R and R" are the same or different alkyl radicals.

5. A smokeless solid propellant composition according to claim 1 which comprises a cured intimate mixture of ammonium perchlorate and a polymerizable monomer having the formula:

RII

wherein R is an alkenyl radical and R' and R" are the same or difierent alkyl radicals.

6. A smokeless solid propellant composition according to claim 1 which comprises a cured intimate mixture of ammonium perchlorate and N,N-dimethyl acrylamide.

7. A smokeless solid propellant composition according to claim 1 which comprises a cured intimate mixture of ammonium nitrate and N,N-dimethyl acrylamide.

8. The composition of claim 6 wherein ammonium perchlorate is present in an amount of from about to about by weight of the total propellant composition.

9. The composition of claim 7 wherein ammonium nitrate is present in an amount of from about 50% to about 80% by weight of the total propellant composition.

10. A smokeless solid propellant composition which comprises about 22.94% N,N-dimethyl acrylamide; 76.81% ammonium nitrate and 0.25% methyl amyl ketone peroxide.

11. The composition of claim 1 to which a polymerizable olefin has been added, said olefin being present in an amount less than about 25% by weight of the total propellant composition.

References Cited in the file of this patent UNITED STATES PATENTS 2,311,548 Jacobson et al. Feb. 16, 1943 2,479,470 Carr Aug. 16, 1949 2,483,194 Gleim Sept. 27, 1949 2,579,426 Gleim Dec. 18, 1951 2,592,254 Dickey Apr. 8, 1952 FOREIGN PATENTS 248,089 Great Britain Mar. 1, 1926 655,585 Great Britain July 25, 1951 

1. A SMOKELESS SOLID PROPELLANT COMPOSITION WHICH COMPRISES A CRUDE INTIMATE MIXTURE OF FROM ABOUT 5% TO ABOUT 95% OF A SOLID INORGANIC OXIDIZING SALT SELECTED FROM THE GROUP CONSISTING OF AMMONIUM, ALKYL AMMONIUM ALKYLARYL AMMONIUM, ALKYLARYL ALKYL AMMONIUM, ANILINIUM, AND ALKYLANILIUM PERCHLORATES, CHLORATES, AND NITRATES, AND FROM ABOUT 95% TO ABOUT 5% OF A POLYMERIZABLE MONOMER HAVING THE FORMULA: 